In a fin-and-tube type heat exchanger such as an air conditioner or a water heater, a heat transfer tube for passing a coolant through the aluminum fin material is provided. In the heat transfer tube, an inner spiral grooved tube having a continuous spiral groove on the inner surface is mainly used for enhancing the heat exchange efficiency with the refrigerant.
Conventionally, copper alloys have been mainly used for heat transfer tubes. However, due to demands for weight saving, cost reduction and improvement in recyclability, there is an increasing demand for the development of heat transfer tubes made of aluminum alloy.
As a method of producing an inner spiral grooved tube (heat transfer tube) made of a copper alloy, a groove rolling method of rolling a spiral groove on the inner surface of at tube is known. However, in a heat transfer tube made of an aluminum alloy, it is necessary to increase the bottom wall thickness in order to increase the pressure resistance, and it was difficult to manufacture by a groove rolling method. Also, in groove rolling, aluminum chip is generated due to friction between the groove plug and the inner surface of the tube, and there is also a problem that it is difficult to remove the chip. Therefore, in order to manufacture an inner spiral grooved tube made of an aluminum alloy, a new production method has been required in place of the groove rolling method.
Patent Literature 1 (PTL 1) discloses an apparatus for producing an inner spiral grooved tube made of an aluminum alloy. In the apparatus, one of one of a winding drum and a rewinding drum is supported by a cradle; and twist is imparted to a tubular material conveyed between the drums by a flyer rotating around the one of the drum